Create a group, if you have a decent distro they'll provide a users/groups gui tool, i know redhat does. Otherwise use groupadd.
Then edit /etc/group, find the group you just added and add the user names of the people in that group to the end of the line.
chown -R you.newgroup thedirectory
That'll mean you own the directory, but it's in the group you just created. Now you can control permissions separately for the user (ie, you) and the group, so if you want the other members of the group to have read only access you can do that. For read/write do
chmod -R g+rw thedirectory/
a+rw is all, g+rw is group, o+rw is owner.